Are there techniques to help reduce the error of estimating the hours required to do a project?
Yes, there are, but none of them will actually guarantee you anything. The main activity of a project manager is planing and tracking. The tracking part is the part where you verify if the previous estimates still hold and, if not, change them.
Beyond this, there ...
How to control my employees productivity?
By looking at what must be done and how they do it.
This requires insight of what they do.
This requires technical understanding of their work.
Not understanding what developers do and what problems they face is not a shame.
But to be honest, it is no good condition for a proper evaluation of what they do.
There are ...
... and I'm noticing their productivity has dropped.
Because of Trello and Jira using. Every week in Trello I can see lesser tasks, than in the other week. And even with my middle knowledge in project management, I can understand they can do much more
The number of issues in JIRA (or whatever) is not at all a good metric for anything. One has to go ...
Before you can start to look for a solution, you need to identify the problem.
First of all, is productivity really down? Burndown charts and ticket system statistics can be deceiving. The individual items might not reflect the real work which was done or is to be done, because they might each represent tasks with varying complexity.
Then assuming it is, you ...
Based on your question, the question you posted, and more important which arguments you seem to only care about to justify your position, I'm severely questioning your project management skills and your ability to lead a team to the project's completion.
Several commenters asked for justification on how you know that productivity is slacking, and your ...